Due to the finite supply of fossil energy sources, the global environmental damage caused by fossil fuels, increasing energy demand, and economic forces, society is becoming compelled to both diversify energy resources and utilize existing fossil fuels more effectively.
One emerging energy resource that has received significant attention is hydrogen. Hydrogen is recognized by many as a key component to a clean, sustainable energy system. Some experts believe that hydrogen may eventually form the basic energy infrastructure to power future societies, replacing today's natural gas, oil, coal, and electricity infrastructures.
Most of the world's current supply of hydrogen is derived from fossil fuels by, for example, steam reforming natural gas, partial oxidation of oil, and water electrolysis using electricity from conventional energy resources. However, it is envisioned that renewable sources, such as biomass, wastes, water, wind, and sun may eventually be cost effectively harnessed to produce hydrogen. A plentiful supply of inexpensive hydrogen from renewable sources may greatly contribute to the diversification of the current energy structure from fossil fuels to hydrogen.
Unfortunately, the lack of practical storage and transport methods has hindered more widespread development and use of hydrogen fuels. The storage and transport of hydrogen can take place in its free form (H2) or chemically bound. Hydrogen is one of the lightest elements and has very small molecules. Thus, the storage of hydrogen in its free form (i.e., liquid hydrogen and compressed hydrogen) is complex because it can escape from tanks and pipes more easily than conventional fuels. To limit hydrogen escape, the fuel must be kept at extremely low temperatures or high pressures which are energy intensive and costly. This hydrogen volatility and low temperature and/or high pressure requirements suggest that no infrastructure for storage and transport of hydrogen in its free form will be developed quickly.
Chemically binding hydrogen usually takes the form of the reaction of a metal with hydrogen to create a metal hydride, although chemical binding with carbonaceous materials to form, for example, methane or methanol has also been explored. Indeed, significant research is being directed toward chemically binding hydrogen with carbonaceous materials to form methane-rich gas because the methane-rich gas can be transported through the existing natural gas pipeline infrastructure.
Although coal is a fossil fuel, it is a relatively plentiful carbonaceous material. Approximately forty percent of the earth's current electricity production is powered by coal. In steam turbines, coal is pulverized, mixed with oxygen, and burned, with the heat producing steam that operates an electric generator. An unfortunate side effect of this process is that many harmful and toxic pollutants are released into the air. In particular, emissions from coal-fired power plants represent the largest source of artificial carbon dioxide emissions. Therefore, with plentiful, inexpensive coal likely to be utilized in the foreseeable future, and during the transition away from fossil fuels, developing more advanced clean coal technologies is essential.
Gasification processes convert coal and other solid fuels to synthesis gas for the production of electricity and transportation fuels. Coal gasification offers one of the most versatile and cleanest ways to convert the energy content of coal into electricity, hydrogen, and other energy forms. Moreover, with a plentiful supply of inexpensive hydrogen, the variant of gasification, i.e., hydrogasification, utilizing the plentiful coal may be a viable option for converting the excess hydrogen to transportable methane-rich gas.
Hydrogasification is an exothermic gasification process in which hydrocarbons are broken down into a methane-rich gas in a hydrogen atmosphere. The main reactions are:C+2H2→CH4 CO+3H2→CH4+H2O
The methane-rich, low tar gas can be passed through a high temperature gas clean-up stage, where contaminants can be removed. Following the gas cleaning operation, the methane-rich gas, also known as synthesis gas (syngas), may be piped to a utility plant.
The primary objective of hydrogasification is to upgrade organic wastes to a methane-rich gas with a low concentration of hydrogen that is readily transportable in a conventional pipeline infrastructure. In addition, this syngas can be burned directly in a combined cycle turbine. Despite its advantages, hydrogasification of carbonaceous materials has not been explored extensively for commercial purposes because the cost of hydrogen is considered prohibitive. In addition, the efficiency for a hydrogasification process to produce methane-rich gas is not yet sufficient to be economically feasible.
The problems to be surmounted in a hydrogasification process and apparatus, assuming a plentiful and inexpensive supply of free form hydrogen, include effective delivery and mixing of the carbonaceous material with hydrogen, and rapid reaction of the carbonaceous material with the hydrogen to form a methane-rich gas that is compatible with existing transportation and storage infrastructures.
Known hydrogasification processes typically utilize water or oil to form a slurry of carbonaceous material that is subsequently conveyed into a reactor. In addition, steam may be introduced into the hydrogasification and react with the char (carbon remains) to produce carbon monoxide and hydrogen, and to provide temperature control. These hydrogasification processes are economically inefficient, require complex and costly equipment, and call for an undesirably large quantity of water which is a commodity in arid regions.